One long standing theory concerning the control of development from fertilized egg into embryo is that maternal molecules, that is molecules present in the egg prior to fertilization, direct the events of early embryogenesis. By localization of maternal molecules to specific regions of the egg, the cells that result from cleavage of the egg inherit different sets of maternal molecules, giving them a unique identity and endowing them with defined developmental capacity. Understanding the functional role of localized maternal molecules is viewed as a way to understand control of early development. There are many experimental results confirming the existence of localized maternal molecules. Recently members of one class of localized molecule, messenger RNA (mRNA) have been identified and cloned. These recent successes encourages the study of localized mRNA as a starting point towards understanding the role of localized maternal molecules. The experimental system that will be used is the frog, Xenopus laevis. Among the questions that will be addressed, is the way the transcription of the genes encoding these mRNA are controlled. The primary sequence of the genes will be examined for similarities and differences, and the putative transcriptional control regions will be studied during oocyte development and embryogenesis to discover the factor that influence both their activation and repression. These studies should lead to a better understanding of what regulates maternal and embryonic transcription. mRNA exert their role by being translated into proteins. The temporal appearance, and embryonic localization of the protein products of the localized mRNAs will also be examined. These proteins will be visualized using antibodies that recognize them, their functions studied both by elucidating their similarity to proteins of known function by structural analysis, and by introduction back into developing embryos to examine their affect when mislocalized, mutated, or in greater abundance than normally found in the embryo. Finally, only a handful of localized mRNAs have been isolated to date. The isolation of new members of this class of maternal molecule, from different regions of the egg and embryo is necessary. These new molecules will be identified and isolated by differential screening of maternal RNA derived cDNA lambda libraries.